a(n) = 2*(2^n + 1)*(2^(n+1) - 1).

Terms

a(0) =4a(1) =18a(2) =70a(3) =270a(4) =1054a(5) =4158a(6) =16510a(7) =65790a(8) =262654a(9) =1049598a(10) =4196350a(11) =16781310a(12) =67117054a(13) =268451838a(14) =1073774590a(15) =4295032830a(16) =17180000254a(17) =68719738878a(18) =274878431230